(1) Field of the Invention
The present invention relates to a nuclear medicine diagnosis device, a form tomography diagnosis device, a nuclear medicine data arithmetic processing method, and a form tomogram arithmetic processing method for obtaining nuclear medicine data or form tomograms of a subject injected with radiopharmaceutical based on radiant rays generated from the subject.
(2) Description of the Related Art
The nuclear medicine diagnosis device, that is, an ECT (Emission Computed Tomography) device will be described while taking a PET (Positron Emission Tomography) device as an example. The PET device is configured to detect a plurality of gamma (γ) rays generated as a result of annihilation of positrons and to reconstruct tomograms of a subject only when a plurality of detectors detects the γ rays simultaneously.
Specifically, a radiopharmaceutical containing a positron-emitting radionuclide is administered into the subject and detectors each constituted by many detector components (such as scintillators) detect pair annihilation γ rays of 511 KeV emitted from within the subject injected with the radiopharmaceutical. If the two detectors detect γ rays in certain time, it is assumed that the detectors detect γ rays simultaneously. The detected γ rays are calculated as pairs of pair annihilation γ rays and a pair annihilation generation point is identified to be on a line of each pair of detectors detecting the γ rays. By accumulating such coincidence count information and performing a reconstruction processing, positron-emitting radionuclide distribution images (that is, tomograms) is obtained. The technique is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 7-113873 and 2000-28727.
To keep quantitative performance and imaging quality high in the nuclear medicine diagnosis, it is essential to absorb and correct the coincidence count information data (also referred to as “emission data”). Absorption of the coincidence counting data by the PET device depends on a path on which the γ rays pass through the subject and does not depend on γ ray generation points (positron pair annihilation generation points). Normally, therefore, an external radiant source irradiating radiant rays of the same type (γ rays in this case) as that of the radiopharmaceutical is employed. An inverse of a transmission factor or an absorption correction value obtained from an absorption coefficient map is multiplied by emission projection data, whereby form information (also referred to as “transmission data”) based on the γ rays irradiated from the external radiant source and transmitted by the subject can be absorption-corrected. Recently, a technique for converting form information obtained from an X-ray CT device integrated with a PET device (PET-CT device) in place of the external radiant source into an absorption coefficient map and for using the absorption coefficient map for absorption correction has been adopted.
However, if it is difficult to mount the external radiant source or the like and an interior of the subject can be assumed as a uniform absorber, a technique for estimating a contour of the subject from the emission data or images, assuming the interior as a uniform absorber and performing absorption correction on the uniform absorber is adopted. This technique is disclosed in, for example, KITAMURA Keiji, ISHIKAWA Yoshihiro, MIZUTA Tetsuro, YOSHIDA Eiji and YAMAYA Taiga: “Development of Various Data Correction Method in jPET-D4”, Next-generation PET Device Research and Development Report 2005, pp. 47 to 51.
Recently, particularly in development of high resolution PET, scintillators (LSO, LYSO, LGSO, etc.) including Lu-176 have been often used as the scintillators constituting each detector in light of a high emission amount, short luminescent decay time and high γ ray blocking capability during conversion of radiant rays into light by the scintillators. These characteristics are the basis for and have influence on performances of the PET device, that is, high resolution (size reduction of each scintillator), high counting rate (accelerated event processing) and high sensitivity (high probability of γ ray detection).
However, the element Lu-176 is a radioactive substance and three γ-decays (300 KeV 94%, 202 KeV 78%, 88 KeV 15%) occur concurrently to follow a beta decay (β-decay) (99.9%, maximum 596 KeV). Due to this, there are cases where a plurality of (two or more) arbitrary radiant rays among these radiant rays is counted coincidentally. This coincidence count cannot be subtracted as “random coincidence count”. However, in collection of data in PET, a energy lower limit threshold (300 to 400 KeV) is normally set so as to remove low energy background such as scatter components, as disclosed in, for example, Andrew L. et al.: “On the imaging of very weak sources in an LSO PET Scanner”, IEEE MIC 2007, Conf Rec. MO7-5, S Yamamoto et al., “Investigation of single, random, and true counts from natural radioactivity in LSO-based clinical PET”, Ann Nucl Med, vol. 19, pp. 109 to 114, 2005. Components other than the γ-rays (511 KeV) are removed from detection target positrons (that is, radiopharmaceutical). It is reported that self-radioactivity of Lu-176 can be suppressed to almost an ignorable level by setting this energy lower limit threshold to about 400 KeV. In this way, the self-radioactivity of Lu-176 may possibly become background noise, so that it is a main conventional object to suppress the components.
Meanwhile, it is necessary to suppress the self-radioactivity during coincidence counting. A technique or the like for daily checking of detectors (each including a photo multiplier tube (PMT) and an electric circuit) using the self-radioactivity is proposed. The technique is disclosed in, for example, Christof Knoess et al.: “Development of Daily Quality Check Procedure for the High-Resolution Research Tomograph (HRRT) Using Natural LSO Background Radioactivity”, IEEE Trans. Nucl. Sci., vol. 49, No. 5, P2074, 2002.
The conventional absorption correction method using the external radiant source and X-ray CT images as stated above is highly accurate and effective. Nevertheless, if the detectors are located to be proximate to the subject with views of improving sensitivity and spatial resolution, a space for mounting a collimated external radiant source, a mechanism that rotates the radiant source (radiant source rotation mechanism) and the like is not often secured. Furthermore, in case of a PET mammography device applied to mammograms for detecting a breast cancer, it is necessary to make a body (breast) of a subject as proximate to the detectors as possible. If an interior of the subject can be considered a uniform absorber, the technique for extracting a profile of the subject from emission data and images, regarding the interior as the uniform absorber and conducting an absorption correction is used. However, if radioactive accumulation is quite small on edges of the subject, the profile cannot be extracted and profile extraction accuracy is deteriorated. Moreover, since distribution is extremely offset on the edges of the subject, the profile extraction accuracy may possibly be deteriorated. In this way, stable profile information cannot be acquired and stable absorption correction cannot be conducted depending on an accumulation situation of the radiopharmaceutical.